Problem: If $4a + b + 5c = -9$ and $7x + 7y = 4$, what is $-45c - 36a + 21y + 21x - 9b$ ?
Solution: $= -36a - 9b - 45c + 21x + 21y$ $= (-9) \cdot (4a + b + 5c) + (3) \cdot (7x + 7y)$ $= (-9) \cdot (-9) + (3) \cdot (4)$ $= 81 + 12$ $= 93$